1. Field of the Invention
The present invention relates to an image forming apparatus adopting the electrophotographic method which forms an electrostatic image on a light-sensitive medium in accordance with a pulse width modulation signal of an image density signal and develops it with toner to perform image formation.
2. Discussion of the Related Art
In a printer or a copying machine, a digital electrophotographic method is widely used as a method capable of realizing high-speed image formation and providing high image quality. In this method, scanning of the light-sensitive medium with light beam and exposure with pulse width modulation using an analog screen generator or the like for reproducing gradation of an image are generally carried out (See Japanese Patent Application Unexamined Publication No. Hei. 1-280965 (1989), for example).
In this method, an image is formed with fixed spot diameter of the light beam and number of lines from a low density portion to a high density portion of the image density signal. Therefore, contrast of an exposure profile in the low density portion is decreased, thus being analog-like, and besides, since the exposure amount itself is small, reproducing capability of dots or lines and stability in reproduction of gradations and tones depending on environments deteriorate.
To overcome the above problem, the contrast of the exposure profile in the low density portion can be increased by making the spot diameter of the light beam sufficiently small. However, an image-forming optical system converging the light beam and forming a light beam spot on the light-sensitive medium is required excessive preciseness and is very expensive; accordingly, it is of no practical use.
However, in the above-described invention, it is ascertained in the case where a mixture of a line image such as a character image and a natural image including a halftone image is tried to be reproduced, neither line image nor natural image is sufficiently reproduced. This is because the number of recording lines (density) suited for reproduction of the line image differs from the number of recording lines suited for reproduction of the natural image.